Blood for diagnostic purposes is presently drawn in a system comprising a glass evacuated tube fitted by a rubber stopper to seal said tube from the outside enviornment. A plastic cylindrical holder is used in conjunction with a special needle with two sharpened ends. One end of said needle is designed to be threaded into one end of holder. A tube can then be inserted within said holder to meet one end of sharp needle. When vena puncture is made the tube within the holder is pressed firmly foward wherein the end on the needle within holder penetrates rubber stopper and allows blood to flow into tube due to negative pressure or evacuation of tube. When the pressure within the tube equals the pressure of blood entering, no blood flows. It can however be demonstrated experimentaly and confirmed by computations that it is possible under certain conditions for backflow to occur during this process especially at the point of removal of tourniquet while needle is still within patient's vein.
It is with this problem in mind that the invention described herein offers for the first time a novel method of eliminating or minimizing the hazards of backflow.